Some current diaphragm-type carburetors utilize engine crankcase pressure pulses applied to the so-called dry side of a carburetor fuel control diaphragm to control or enrich the carburetor fuel and air mixture delivered to an engine during starting and warming up of the engine. The application of the engine crankcase pressure pulses in these current carburetors, such as disclosed in U.S. Pat. No. 4,814,114, is controlled by a manually operated, three-position valve. The valve has a fully closed position, a fully open position and an intermediate position between the fully closed and fully open positions.
To start an engine having this type of carburetor, air is purged from the carburetor, such as by depressing an air purge bulb, the throttle valve is moved to its fully opened or wide open throttle position and the three position valve is moved to its fully open position permitting engine crankcase pressure pulses to act on the fuel control diaphragm. The operator then tries to manually start the engine such as by pulling an engine starter rope or cord until engine combustion is initiated but not normally sustained and the engine stalls. The valve is then moved to its intermediate position decreasing the application of engine crankcase pressure pulses to the fuel control diaphragm. The operator continues to try and start the engine until the engine is started and operation of the engine is sustained. After a short period of time sufficient to allow the engine to warm up, the valve is turned to its fully closed position preventing the application of engine crankcase pressure pulses to the fuel control diaphragm. Starting an engine having a carburetor with this manual three-position choke valve can be difficult for unskilled operators who are unfamiliar with the multi-step engine starting process required with this type of carburetor. Further, the starting procedure has to be modified under different temperature conditions and the operator must have the knowledge and skill to employ the necessary starting procedure.